Neuromyelitis optica (NMO) is currently the best defined acquired inflammatory demyelinating disorder of the central nervous system (CNS). NMO attacks optic nerves and spinal cord selectively and repeatedly. Clinical, histopathological and immunobiological observations support a pathogenic role for an IgG autoantibody specific for the astrocytic water channel aquaporin-4 (AQP4), and the severity of acute NMO is ameliorated by antibody-depleting therapies.
In contrast to most inflammatory CNS demyelinating disorders, tissue destruction in NMO is profound. In addition to white matter lesions, NMO characteristically exhibits central necrosis of spinal cord gray matter. Histopathological CNS lesions lack AQP4 and show deposition of IgM and IgG and products of complement activation in a vasculocentric pattern that coincides with the normal distribution of AQP4.
Until recently, NMO was considered a rare and severe variant of multiple sclerosis (MS). However, the advent of serological testing for AQP4-IgG has revealed that NMO and its inaugural forms are more common than previously recognized. They tend to be misdiagnosed as MS, which lacks a specific biomarker.